1. Field of the Invention
The present invention relates to an image processing apparatus, image processing method, and storage medium used to generate a color conversion table required to correct colors of an image output from an output device.
2. Description of the Related Art
In recent years, performances of printers which form images using an electrophotography system have been improved, and models which realize image quality equivalent to a printing press have appeared. However, such printers suffer problems of larger variation amounts of formed colors than a printing press due to instability unique to the electrophotography system. In order to suppress variation amounts, a conventional electrophotographic printer executes calibrations of unicolors corresponding to cyan (C), magenta (M), yellow (Y), and black (K) toners.
However, execution of only the calibrations of unicolors cannot often sufficiently correct colors since the transfer efficiency, fixing efficiency, and the like of the printer change upon formation of mixed colors. In this case, the transfer efficiency is a non-losing ratio of toners upon transferring toners of respective colors applied on a transfer belt onto a print sheet. For example, when the transfer efficiency of a unicolor (cyan) is 100, that at the time of a mixed color may become 80. Hence, even when colors are matched as unicolors, they may be different at the time of mixed colors. Note that the same applies to the fixing efficiency.
Hence, in recent years, a technique for executing calibrations of mixed colors such as red (R), green (G), blue (B), black (K), and the like has been proposed. For example, a technique for correcting colors of the mixed colors by updating a three-dimensional lookup table (3D-LUT) used to convert data on a device-independent color space (L*a*b*) onto a device-dependent color space (CMYK) is available. With this technique, an image using chart data generated based on mixed colors is output by a printer, and is measured by a scanner or colorimeter. Differences between colorimetric results and target values are calculated in an L*a*b* color space. In this case, the differences are calculated as vectors having magnitudes and directions in the L*a*b* color space. Then, an arbitrary number of differences are extracted in ascending order of distance from a grid point of interest in the L*a*b* color space, and a correction amount is calculated based on the extracted differences. This processing is repeated for all grid points. The reason why the plurality of differences are extracted is to grasp variations between the grid point of interest and surrounding points precisely.
The calculated correction amount has a magnitude and direction in the L*a*b* color space, and indicates a position to which each grid point is to be moved on the L*a*b* color space, that is, an output value for an input L*a*b* value, so as to correct color variations. By correcting all the grid points based on the correction amounts, the 3D-LUT which converts data on the L*a*b* color space onto the CMY color space can be updated, that is, colors of the mixed colors can be corrected.
Also, a technique for executing calibrations of mixed colors using smaller information volumes by generating chart data optimal to a device based on information such as differences between colorimetric results and target values has been proposed (for example, see Japanese Patent Laid-open No. 2004-015643).
However, when a chart image is measured during mixed color calibrations, if a variation amount of a color corresponding to a certain patch is large, differences between colorimetric results and target values on the L*a*b* color space may be large, or a certain patch and neighboring patches may have different directions of changes (differences) between colorimetric results and target values. Such outstanding differences may not precisely reflect variations of colors. When the 3D-LUT is to be corrected during calibrations, grid points are moved with reference to such differences to correct their output values. However, when a certain grid point refers to an outstanding difference, a moving amount of that grid point, that is, a correction amount of an output value may be improper. Also, when moving amounts of only some grid points are large, distortions are generated in color reproduction using the corrected LUT, resulting in a tonality drop sifter color conversion. The same phenomenon is likely to occur when improper chart data is used. In order to suppress any tonality drop caused by the 3D-LUT correction, it is required to generate proper chart data.